test microdroid vendor images: set avb_hash_algorithm to "sha256"

When not specified the avb_hash_algorithm defaults to "sha1", which is
discouraged in Android. This also blocks the updated to microdroid
kernel prebuilts (the sha1 support was recently removed from the
microdroid_defconfig).

The aosp/50fb49cabe70cfbde6743acb18543cd14befb7a5 changes build system
to default avb_sha_algorithm to "sha256", however it might take some
time for that patch to be reviewed, hence manually setting the value
here.

Test: atest MicrodroidTests
Change-Id: I67fb4512b65dcc76843d0a426517b8ef9cc67339
diff --git a/tests/vendor_images/Android.bp b/tests/vendor_images/Android.bp
index 0430eaa..c4cbcb7 100644
--- a/tests/vendor_images/Android.bp
+++ b/tests/vendor_images/Android.bp
@@ -9,6 +9,7 @@
     file_contexts: ":microdroid_vendor_file_contexts.gen",
     use_avb: true,
     avb_private_key: ":avb_testkey_rsa4096",
+    avb_hash_algorithm: "sha256",
     rollback_index: 5,
 }
 
@@ -18,6 +19,7 @@
     type: "ext4",
     file_contexts: ":microdroid_vendor_file_contexts.gen",
     use_avb: true,
+    avb_hash_algorithm: "sha256",
     avb_private_key: ":avb_testkey_rsa4096",
 }